Head over to Lirene's Ferldan Imports and talk to her about the Grey Warden, then head to Darktown. Talk to Anders in the northeast corner of Darktown. He wants you to rescue his friend before he'll help you.

Head to the Chantry in Hightown and follow the minimap to the battle. Anders is available after this as a companion, his strength is healing. He'll also give you the maps you need for Bartrand, which ends the quest.